The petrochemical industry has long used naturally forming hydrocarbon feedstocks for the production of valuable olefinic materials, such as ethylene and propylene. Ideally, commercial operations have been carried out using normally gaseous hydrocarbons such as ethane and propane as the feedstock. As the lighter hydrocarbons have been consumed and the availability of the lighter hydrocarbons has decreased, the industry has more recently been required to crack heavier hydrocarbons. Hydrocarbons such as naphtha, atmospheric gas oils (AGO) and vacuum gas oils (VGO) which have higher boiling points than the gaseous hydrocarbons are being used commercially.
A typical process for the production of olefins from hydrocarbon feedstocks is the thermal cracking process. In this process, hydrocarbons undergo cracking at elevated temperatures to produce hydrocarbons containing from 1 to 4 carbon atoms, especially the corresponding olefins.
At present, there are a variety of processes available for cracking heavier hydrocarbons to produce olefins. Typically, the hydrocarbon to be cracked is delivered to a furnace comprised of both a convection and radiant zone. The hydrocarbon is initially elevated in temperature in the convection zone to temperatures below those at which significant reaction is initiated; and thereafter is delivered to the radiant zone wherein it is subjected to intense heat from radiant burners. An example of a conventional furnace and process is shown in U.S. Pat. No. 3,487,121 (Hallee).
Illustratively, process fired heaters are used to provide the requisite heat for the reaction. The feedstock flows through a plurality of coils within the fired heater, the coils being arranged in a manner that enhances the heat transfer to the hydrocarbon flowing through the coils. The cracked effluent is then quenched either directly or indirectly to terminate the reaction. In conventional coil pyrolysis, dilution steam is used to inhibit coke formation in the cracking coil. However, in the production of the olefins from hydrocarbon feedstocks the generation of coke has been a problem regardless of the process used. Typically, the cracking reaction will cause production of pyrolysis fuel oil, a precursor to tar and coke materials which foul the equipment. A further benefit of steam dilution is the inhibition of the coke deposition in the heat exchangers used to rapidly quench the cracking reaction.
More recently, the thermal cracking process has been conducted in an apparatus which allows the hydrocarbon feedstock to pass through a reactor in the presence of steam while employing heated particulate solids as the heat carrier. After cracking, the effluent is rapidly quenched to terminate the cracking reactions, the solids being separated from the effluent, preheated and recycled.
In the past, when light hydrocarbons, ethane to naphtha, were used to produce olefins in the thermal cracking process these hydrocarbons could be cracked with dilution steam in the range of 0.3 to 0.6 pounds of steam per pound of hydrocarbon. Heavy hydrocarbons require from about 0.7 to 1.0 pounds of dilution steam per pound of hydrocarbon. As a general proposition, the higher quantities of dilution steam are needed for heavier hydrocarbons to obtain the desired partial pressure of the hydrocarbon stream that is required to suppress the coking rates in the radiant coils during thermal cracking. Correlatively, the dilution steam requirement demands increased furnace size and greater utility usage.
It is well-known that in the process of cracking hydrocarbons, the reaction temperature and reaction residence time are two primary variables affecting severity, conversion and selectivity. Severity is related to the intensity of the cracking reactions. It is related to the reaction velocity constant of n-pentane in reciprocal seconds and the time (t) in seconds. Conversion is the measure of the extent to which the feed has been pyrolyzed (% to which n-pentane would have decomposed under the history of the feed). Conversion of commercial hydrocarbon feeds has been related to the conversion of normal pentane (c) by the following expression: EQU Kt=1n [c/(100-c)]
wherein K is the reaction velocity constant of normal pentane in reciprocal seconds, doubling about every 20.degree. F.; and t is the reaction time in seconds.
Selectivity is the degree to which the converted products constitute ethylene. Selectivity is generally expressed as a ratio of olefin products to fuel products.
At low severity, selectivity is high, but because conversion is low, it is uneconomical to utilize a low severity operation. Low severity operation is conducted generally at temperatures between 1200 and 1400.degree. F. and residence times between 2000 and 10000 milliseconds. High severity and high conversion may be achieved at temperatures between 1500.degree. F. and 2000.degree. F. However, selectivity is generally poor at temperatures above 1500.degree. F. unless the high severity reaction can be performed at residence times below 200 milliseconds, usually between 20 and 100 milliseconds. At these very low residence times selectivities between 2.5 and 4.0 pounds of ethylene per pound of methane can be achieved, and conversion is generally over 95% by weight of feed. High severity operation, although preferred, has not been employed widely in the industry because of the physical limitations of conventional fired reactors. One of the limitations is the inability to remove heat from the product effluent within the allowable residence time parameter. For this reason, most conventional systems operate at conditions of moderate severity, temperatures being between 1350.degree. and 1550.degree. F. and residence times being between 200 and 500 milliseconds. Although conversion is higher than at the low severity operation, selectivity is low, being about two pounds of ethylene per pound of methane. But because conversion is higher the actual yield of ethylene is greater than that obtained in the low severity operation.
The yield of pyrolysis fuel oil (PFO) increases with conversion. The rate of formation of PFO increases dramatically above a critical conversion level, where the critical conversion level is a function of feed quality. It occurs at about 75% conversion of heavy naphtha and 85% conversion for lighter naphtha.
By using low residence time at high severity conditions, it is possible to achieve selectivities of about 3:1 or greater. As a result, a number of processes have been developed which offer high severity thermal cracking. For example, furnaces have been developed which contain a large number of small tubes wherein the outlet of each tube is connected directly to an individual indirect quench boiler. This process has the disadvantage of being capital intensive in that the quench boiler is not common to a plurality of furnace tube outlets. Thus, the number of quench boilers required increases. Further, the high temperature waste heat must be used to generate low temperature, high pressure steam which is not desirable from a thermal efficiency viewpoint. Finally, high flue gas temperatures must be reduced by generation of steam in the convection section of the heater, again limiting the flexibility of the process.
In Hallee, U.S. Pat. No. 3,407,789, the furnace comprised a convection preheat zone and a radiant conversion zone or cracking zone. In the radiant section, the conduits or tubes through which the fluid to be treated passes are of relatively short length and small-diameter and of low pressure drop design. The quenching zone is close coupled to the reaction products outlet from the furnace and provides rapid cooling of the effluent from the reaction temperature down to a temperature at which the reaction is substantially stopped and can be cooled further by conventional heat exchange means.
Thus, as reaction time is reduced, it is necessary to increase the process temperature (P) in order to maintain a desired conversion level. It is generally accepted that selectivity and yield increase as residence time is reduced. Industrial plants built to reduce residence times to about 100 milliseconds, however, have run into several obstacles. The run lengths, the period between coil decokings, are reduced from several months to several days. In addition, capital and fuel costs have both increased.
The relations available to the reactor designer to reduce reaction time are summarized by the following equation: ##EQU1## where D=coil i.d. measured in feet
H=heat absorbed in the radiant reactor in BTU/lb PA1 d=density of process fluid in lb/ft.sup.3 PA1 Q=heat flux in BTU/(sec ft.sup.2)
For conventional plants there is little opportunity to reduce D, H or d. D is set by practical limitations in the fabrication of long heat resistant alloy tubes. H is controlled by nature and is equal to the amount of energy required to achieve a given feed conversion. The process fluid density, d, is primarily set by the minimum practical pressure at the coil outlet. Increasing the remaining variable Q, heat flux, increases the difference between metal (M) and process (P) temperatures.
It has already been pointed out that reducing t requires an increase in P. Thus, reducing t increases both M and P, the increase in M being compounded. Increasing either M or P increases the rate of coke deposition. Both of these factors are further exacerbated by the common industrial practice of maximizing conversion in the radiantly heated coil, and by minimizing conversion in the tie line between the coil outlet and the quench boiler inlet.
Increasing Q requires an increase in the temperature of the radiant firebox, thus increasing the BTU of fuel per BTU of H, raising fuel costs per pound of olefin produced.
It would therefore satisfy a long felt need in the art if a pyrolysis system could be provided which maintains a 2 to 3 month run length at improved thermal efficiency and lower capital costs with a significant reduction in t.
Surprisingly, applicants have found that contrary to the teachings of the prior art that conditions used for conversion of normally liquid hydrocarbons below 10 to 20% have little or no effect on olefin yield or selectivity; that the yield of pyrolysis fuel oil, a precursor of coke, increases rapidly above a critical severity, conversions of 65 to 75%; that the temperature profile used for reaction has no measurable influence on yield or selectivity provided the target conversion is reached in the same time and at the same pressure level; and that the maximum metal temperature at a given radiant firebox temperature can be reduced by decreasing the radiant beam length with little or no influence on reaction time at conversion levels above about 50 percent.